Display apparatus using a wire grid polarizing beamsplitter with compensator

ABSTRACT

A projection display apparatus ( 10 ) uses a modulation optical system ( 200 ) that includes a liquid crystal device ( 210 ) in combination with a wire grid polarization beamsplitter ( 240 ), a wire grid polarization analyzer ( 270 ) and a projection lens ( 285 ) to form an image. In order to achieve high contrast and maintain high brightness levels, a compensator ( 260 ) is provided for minimizing leakage light for pixels in the black (OFF) state. A number of configurations are possible, such as with the compensator ( 260 ) disposed in the optical path between the liquid crystal device ( 210 ) and the wire grid polarization beamsplitter ( 240 ) and with a secondary compensator ( 265 ) disposed between wire grid polarization analyzer ( 270 ) and the wire grid polarization beamsplitter ( 240 ).

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] Reference is made to commonly-assigned copending U.S. patentapplication Ser. No. 09/813,207, filed Mar. 20, 2001, entitled DIGITALCINEMA PROJECTOR, by Kurtz et al., the disclosure of which isincorporated herein.

FIELD OF THE INVENTION

[0002] This invention generally relates to digital projection apparatusemploying liquid crystal devices for image forming and more particularlyto an apparatus and method for achieving high levels of contrast byusing a wire grid polarization beamsplitter with a compensator forminimizing leakage light in the pixel black (OFF) state.

BACKGROUND OF THE INVENTION

[0003] In order to be considered as suitable replacements forconventional film projectors, digital projection systems must meetdemanding requirements for image quality. This is particularly true forcinematic projection systems. To provide a competitive alternative toconventional cinematic-quality projectors, digital projection apparatus,provide high resolution, wide color gamut, high brightness (>10,000screen lumens), and frame-sequential system contrast ratios exceeding1,000:1.

[0004] The most promising solutions for digital cinema projection employone of two types of spatial light modulators as image forming devices.The first type of spatial light modulator is the digital micromirrordevice (DMD), developed by Texas Instruments, Inc., Dallas, Tex. DMDdevices are described in a number of patents, for example U.S. Pat. Nos.4,441,791; 5,535,047; 5,600,383 (all to Hombeck); and U.S. Pat. No.5,719,695 (Heimbuch). Optical designs for projection apparatus employingDMDs are disclosed in U.S. Pat. Nos. 5,914,818 (Tejada et al.);5,930,050 (Dewald); 6,008,951 (Anderson); and 6,089,717 (Iwai). AlthoughDMD-based projectors demonstrate some capability to provide thenecessary light throughput, contrast ratio, and color gamut, currentresolution limitations (1024×768 pixels) and high component and systemcosts have restricted DMD acceptability for high-quality digital cinemaprojection.

[0005] The second type of spatial light modulator used for digitalprojection is the liquid crystal device (LCD). The LCD forms an image asan array of pixels by selectively modulating the polarization state ofincident light for each corresponding pixel. At high resolution, largearea LCDs can be fabricated more readily than DMDs. LCDs are a viablealternative modulator technology to be used in digital cinema projectionsystems. Among examples of electronic projection apparatus that utilizeLCD spatial light modulators are those disclosed in U.S. Pat. Nos.5,808,795 (Shimomura et al.); 5,798,819 (Hattori et al.); 5,918,961(Ueda); 6,010,121 (Maki et al.); and 6,062,694 (Oikawa et al.).Recently, JVC demonstrated an LCD-based projector capable ofhigh-resolution (providing 2,000×1280 pixels), high frame sequentialcontrast (in excess of 1000:1), and high light throughput (nominally, upto 12,000 lumens). This system utilized three vertically aligned (VA)(also referred as homeotropic) LCDs (one per color) driven or addressedby cathode ray tubes (CRTs). While this system demonstrated thepotential for an LCD based digital cinema projector, system complexityand overall reliability remain concerns. In addition, costconsiderations render such a system not yet ready for broadcommercialization in the digital cinema projection market.

[0006] JVC has also developed a new family of vertically aligned LCDs,which are directly addressed via a silicon backplane (LCOS), rather thanindirectly by a CRT. While these new devices are promising, they havenot yet been demonstrated to fully meet the expectations for digitalcinema presentation. The JVC LCD devices are described, in part, in U.S.Pat. Nos. 5,652,667 (Kuragane); 5,767,827 (Kobayashi et al.); and5,978,056 (Shintani et al.) In contrast to early twisted nematic orcholesteric LCDs, vertically aligned LCDs promise to provide much highermodulation contrast ratios (in excess of 2,000:1). U.S. Pat. No.5,620,755 (Smith et al.), also assigned to JVC, specifically describes amethodology for inducing vertical alignment in LC displays. It isinstructive to note that, in order to obtain on screen frame sequentialcontrast of 1,000:1 or better, the entire system must produce >1000:1contrast, and both the LCDs and any necessary polarization optics musteach separately provide ˜2,000:1 contrast. Notably, while polarizationcompensated vertically aligned LCDs can provide contrast >20,000:1 whenmodulating collimated laser beams, these same modulators may exhibitcontrasts of 500:1 or less when modulating collimated laser beamswithout the appropriate polarization compensation. Modulation contrastis also dependent on the spectral bandwidth and angular width (F#) ofthe incident light, with contrast generally dropping as the bandwidth isincreased or the F# is decreased. Modulation contrast within LCDs canalso be reduced by residual de-polarization or mis-orientingpolarization effects, such as thermally induces stress birefringence.Such effects can be observed in the far field of the device, where thetypically observed “iron cross” polarization contrast pattern takes on adegenerate pattern.

[0007] As is obvious to those skilled in the digital projection art, theoptical performance provided by LCD based electronic projection systemis, in large part, defined by the characteristics of the LCDs themselvesand by the polarization optics that support LCD projection. Theperformance of polarization separation optics, such as polarizationbeamsplitters, pre-polarizers, and polarizer/analyzer components is ofparticular importance for obtaining high contrast ratios. The precisemanner in which these polarization optical components are combinedwithin a modulation optical system of a projection display can also havesignificant impact on the final resultant contrast.

[0008] The most common conventional polarization beamsplitter solution,which is used in many projection systems, is the traditional MacNeilleprism, disclosed in U.S. Pat. No. 2,403,731. This device has been shownto provide a good extinction ratio (on the order of 300:1). However,this standard prism operates well only with incident light over alimited range of angles (a few degrees). Because the MacNeille prismdesign provides good extinction ratio for one polarization state only, adesign using this device must effectively discard half of the incominglight when this light is from an unpolarized white light source, such asfrom a xenon or metal halide arc lamp.

[0009] Conventional glass polarization beamsplitter design, based on theMacNeille design, has other limitations beyond the limited angularresponse, which could impede its use for digital cinema projection. Inparticular, bonding techniques used in fabrication or thermal stress inoperation, can cause stress birefringence, in turn degrading thepolarization contrast performance of the beamsplitter. These effects,which are often unacceptable for mid range electronic projectionapplications, are not tolerable for cinema projection applications. Thethermal stress problem has recently been improved upon, with the use ofa more suitable low photo-elasticity optical glass, disclosed in U.S.Pat. No. 5,969,861 (Ueda et al.), which was specially designed for usein polarization components. Unfortunately, high fabrication costs anduncertain availability limit the utility of this solution. Furthermore,while it would be feasible to custom melt low-stress glass prisms suitedto each wavelength band in order to minimize stress birefringence, whilesomewhat expanding angular performance, such a solution is costly anderror-prone. As a result of these problems, the conventional MacNeillebased glass beamsplitter design, which is capable of the necessaryperformance for low to mid-range electronic projection systems,operating at 500-5,000 lumens with approximately 800:1 contrast, likelyfalls short of the more demanding requirements of full-scale commercialdigital cinema projection.

[0010] Other polarization beamsplitter technologies have been proposedto meet the needs of an LCD based digital cinema projection system. Forexample, the beamsplitter disclosed in U.S. Pat. No. 5,912,762 (Li etal.), which comprises a plurality of thin film layers sandwiched betweentwo dove prisms, attempts to achieve high extinction ratios for bothpolarization states. Theoretically, this beamsplitter device is capableof extinction ratios in excess of 2,000:1. Moreover, when designed intoa projection system with six LCDs (two per color), this prism couldboost system light efficiency by allowing use of both polarizations.However, size constraints and extremely tight coating tolerances presentsignificant obstacles to commercialization of a projection apparatususing this beamsplitter design.

[0011] As another conventional solution, some projector designs haveemployed liquid-immersion polarization beamsplitters. Liquid-filledbeamsplitters (see U.S. Pat. No. 5,844,722 (Stephens), for example) havebeen shown to provide high extinction ratios needed for high-contrastapplications and have some advantages under high-intensity lightconditions. However, these devices are costly to manufacture, must befabricated without dust or contained bubbles and, under conditions ofsteady use, have exhibited a number of inherent disadvantages. Among thedisadvantages of liquid-immersion polarization beamsplitters arevariations in refractive index of the liquid due to temperature,including uneven index distribution due to convection. Leakage riskpresents another potential disadvantage for these devices.

[0012] Wire grid polarizers have been in existence for many years, andwere primarily used in radio-frequency and far infrared opticalapplications. Use of wire grid polarizers with visible spectrum lighthas been limited, largely due to constraints of device performance ormanufacture. For example, U.S. Pat. No. 5,383,053 (Hegg et al.)discloses use of a wire grid beamsplitter in a virtual image displayapparatus. In the Hegg et al. disclosure, an inexpensive wire gridbeamsplitter provides high light throughput efficiency when comparedagainst conventional prism beamsplitters. The polarization contrastprovided by the wire grid polarizer of Hegg et al. is very low (6.3:1)and unsuitable for digital projection. A second wire grid polarizer forthe visible spectrum is disclosed in U.S. Pat. No. 5,748,368 (Tamada).While the device discussed in this patent provides polarizationseparation, the contrast ratio is inadequate for cinematic projectionand the design is inherently limited to rather narrow wavelength bands.

[0013] Recently, as is disclosed in U.S. Pat. Nos. 6,122,103 (Perkins etal.); 6,243,199 (Hansen et al.); and 6,288,840 (Perkins et al.), highquality wire grid polarizers and beamsplitters have been developed forbroadband use in the visible spectrum. These new devices arecommercially available from Moxtek Inc. of Orem, Utah. While existingwire grid polarizers, including the devices described in U.S. Pat. Nos.6,122,103 and 6,243,199 may not exhibit all of the necessary performancecharacteristics needed for obtaining the high contrast required fordigital cinema projection, these devices do have a number of advantages.When compared against standard polarizers, wire grid polarizationdevices exhibit relatively high extinction ratios and high efficiency.Additionally, the contrast performance of these wire grid devices alsohas broader angular acceptance (NA or numerical aperture) and morerobust thermal performance with less opportunity for thermally inducedstress birefringence than standard polarization devices. Furthermore,the wire grid polarizers are robust relative to harsh environmentalconditions, such as light intensity, temperature, and vibration. Thesedevices perform well under conditions of different color channels, withthe exception that response within the blue light channel may requireadditional compensation.

[0014] Wire grid polarization beamsplitter (PBS) devices have beenemployed within some digital projection apparatus. For example, U.S.Pat. No. 6,243,199 (Hansen et al.) discloses use of a broadband wiregrid polarizing beamsplitter for projection display applications. U.S.Pat. No. 6,234,634 (also to Hansen et al.) discloses a wire gridpolarizing beamsplitter that functions as both polarizer and analyzer ina digital image projection system. U.S. Pat. No. 6,234,634 states thatvery low effective F#'s can be achieved using wire grid PBS, with someloss of contrast, however. Notably, U.S. Pat. No. 6,234,634 does notdiscuss how polarization compensation may be used in combination withwire grid devices to reduce light leakage and boost contrast for fastoptical systems operating at low F#'s.

[0015] In general, wire grid polarizers have not yet been satisfactorilyproven to meet all of the demanding requirements imposed by digitalcinema projection apparatus, although progress is being made.Deficiencies in substrate flatness, in overall polarization performance,and in robustness at both room ambient and high load conditions havelimited commercialization of wire grid polarization devices forcinematic projection.

[0016] Of particular interest and relevance for the apparatus andmethods of the present invention, it must be emphasized thatindividually neither the wire grid polarizer, nor the wire gridpolarization beamsplitter, provide the target polarization extinctionratio performance (nominally >2,000:1) needed to achieve the desiredprojection system frame sequential contrast of 1,000:1 or better,particularly at small F#'s (<F/3.5). Rather, both of these componentsprovide less than ˜1,200:1 contrast under the best conditions.Significantly, performance falls off further in the blue spectrum.Therefore, to achieve the desired 2,000:1 contrast target for theoptical portion of the system (excluding the LCDs), it is necessary toutilize a variety of polarization devices, including possibly wire gridpolarization devices, in combination within a modulation optical systemof the projection display. However, the issues of designing an optimizedconfiguration of polarization optics, including wire grid polarizers, incombination with the LCDs, color optics, and projection lens, have notbeen completely addressed either for electronic projection in general,or for digital cinema projection in particular. Moreover, the prior artdoes not describe how to design a modulation optical system for aprojection display using both LCDs and wire grid devices, which furtherhas polarization compensators to boost contrast.

[0017] There are numerous examples of polarization compensatorsdeveloped to enhance the polarization performance of LCDs generally, andvertically aligned LCDs particularly. In an optimized system, thecompensators are simultaneously designed to enhance the performance ofthe LCDs and the polarization optics in combination. These compensatorstypically provide angular varying birefringence, structured in aspatially variant fashion, to affect polarization states in portions(within certain spatial and angular areas) of the transiting light beam,without affecting the polarization states in other portions of the lightbeam. Polarization compensators have been designed to work with LCDsgenerally, but also vertically aligned LCDs in particular. U.S. Pat. No.4,701,028 (Clerc et al.) discloses birefringence compensation designedfor a vertically aligned LCD with restricted thickness. U.S. Pat. No.5,039,185 (Uchida et al.) discloses a vertically aligned LCD withcompensator comprising at least two uniaxial or two biaxial retardersprovided between a sheet polarizer/analyzer pair. U.S. Pat. No.5,298,199 (Hirose et al.) discloses the use of a biaxial filmcompensator correcting for optical birefringence errors in the LCD, usedin a package with crossed sheet polarizers, where the LCD dark state hasa non-zero voltage (a bias voltage). U.S. Pat. No. 6,081,312 (Aminaka etal.) discloses a discotic film compensator which is designed to optimizecontrast for a voltage ON state of the VA LCD. By comparison, U.S. Pat.No. 6,141,075 (Ohmuro et al.) discloses a VA LCD compensated by tworetardation films, one with positive birefringence and the other withnegative birefringence.

[0018] U.S. Pat. No. 5,576,854 (Schmidt et al.) discloses a compensatorconstructed for use in projector apparatus using an LCD with theconventional MacNeille prism type polarization beamsplitter. Thiscompensator comprises a ¼ wave plate for compensating the prism and anadditional 0.02 λ's compensation for the inherent LCD residualbirefringence effects. U.S. Pat. No. 5,619,352 (Koch et al.) disclosescompensation devices, usable with twisted nematic LCDs, where thecompensators have a multi-layer construction, using combinations ofA-plates, C-plates, and O-plates, as needed.

[0019] In general, most of these prior art compensator patents assumethe LCDs are used in combination with sheet polarizers, and correct forthe LCD polarization errors. However, polarization compensators havealso been explicitly developed to correct for non-uniform polarizationeffects from the conventional Polaroid type dye sheet polarizer. The dyesheet polarizer, developed by E. H. Land in 1929 functions by dichroism,or the polarization selective anistropic absorption of light.Compensators for dye sheet polarizers are described in Chen et al. (J.Chen, K.-H. Kim, J.-J. Kyu, J. H. Souk, J. R. Kelly, P. J. Bos, “OptimumFilm Compensation Modes for TN and VA LCDs”, SID 98 Digest, pgs.315-318.), and use a combination A-plate and C-plate construction. Themaximum contrast of the LCD system aimed at in prior art patents such asin U.S. Pat. No. 6,141,075 (Ohmuro et al.) is only up to 500:1, which issufficient for many applications, but does not meet the requirement ofdigital cinema projection.

[0020] While this prior art material extensively details the design ofpolarization compensators used under various conditions, compensatorsexplicitly developed and optimized for use with wire grid polarizers arenot disclosed. Furthermore, the design of polarization compensators toenhance the performance of a modulation optical system using multiplewire grid polarizer devices, or using multiple wire grid devices incombination with vertically aligned LCDs, have not been previouslydisclosed. Without compensation, the wire grid polarization beamsplitterprovides acceptable contrast when incident light is within a lownumerical aperture. However, in order to achieve high brightness levels,it is most advantageous for an optical system to have a high numericalaperture (>˜0.13), so that it is able to gather incident light at largeroblique angles. The conflicting goals of maintaining high brightness andhigh contrast ratio present a significant design problem forpolarization components. Light leakage in the OFF state must be minimalin order to achieve high contrast levels. Yet, light leakage is mostpronounced for incident light at the oblique angles required forachieving high brightness.

[0021] Compensator requirements for wire grid polarizing beamsplitterdevices differ significantly from more conventional use of compensatorswith polarizing beamsplitter devices based on the MacNeille prism designas was noted in reference to U.S. Pat. No. 5,576,854. Performanceresults indicate that the conventional use of a ¼ wave plate compensatoris not a solution and can even degrade contrast ratio. Additionally,while compensators have previously been specifically developed to workin tandem with VA LCDs in projection display systems, compensatorsoptimized for use with VA LCDs in the context of a modulation opticalsystem which utilizes wire grid polarization beamsplitters have not beendeveloped and disclosed.

[0022] Thus it can be seen that there is a need for an improvedprojection apparatus that uses wire grid polarization devices,vertically aligned LCDs, and polarization compensators in combination toprovide high-contrast output.

SUMMARY OF THE INVENTION

[0023] Briefly, according to one aspect of the present invention adisplay apparatus comprises a light source for forming a beam of light.A pre-polarizer polarizes the beam of light to provide a polarized beamof light. A wire grid polarizing beamsplitter receives the polarizedbeam of light. The polarized beam of light transmits a firstpolarization for reflecting the polarized beam of light that has asecond polarization. A reflective liquid crystal device selectivelymodulates the polarized beam of light that has a first polarization toencode image data in order to form a modulated beam, and for reflectingthe modulated beam back to the wire grid polarizing beamsplitter. Acompensator is located between the wire grid polarization beamsplitterand the reflective liquid crystal device. The liquid crystal deviceconditions oblique and skew rays of the modulated beam reflected fromthe wire grid polarizing beamsplitter. The wire in the polarizingbeamsplitter reflects the compensated modulated beam. Image-formingoptics form an image from the modulated beam.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] While the specification concludes with claims particularlypointing out and distinctly claiming the subject matter of the presentinvention, it is believed that the invention will be better understoodfrom the following description when taken in conjunction with theaccompanying drawings, wherein:

[0025]FIG. 1 is a schematic view showing an arrangement of opticalcomponents in a projection apparatus;

[0026]FIG. 2 is a perspective view of a prior art wire grid polarizer.

[0027]FIG. 3 is a cross sectional view showing a modulation opticalsystem which includes a wire grid polarization beamsplitter.

[0028]FIG. 4 is a graph showing the relationship of contrast ratio toF/# for a modulation optical system which includes both a wire gridpolarization beamsplitter and an LCD, both with and without polarizationcompensation.

[0029]FIG. 5a shows the geometry of incident light relative to the wiregrid polarizing beamsplitter and an LCD within a modulation opticalsystem, illustrating both polarization states and the local beamgeometry.

[0030]FIG. 5b illustrates the geometry of normally incident lightrelative to the polarization states of crossed polarizers.

[0031]FIG. 5c illustrates the geometry of an unfolded modulation opticalsystem with a transmissive spatial light modulator, wire gridpolarizers, and a polarization compensator.

[0032]FIGS. 6a and 6 b show the angular response for crossed wire gridpolarizers without polarization compensation.

[0033]FIGS. 7a-e show the possible axial orientations and constructionof a polarization compensator.

[0034]FIGS. 8a-i are the far field angular response plots from variousarrangements of wire grid polarization devices and compensators.

[0035]FIG. 9a shows the contrast contour plot for an ideal VA LCDwithout compensator.

[0036]FIG. 9b shows the contrast contour plot for a VA LCD with 10 nminduced retardation from ITO substrate.

[0037]FIG. 9c shows the contrast contour plot for a VA LCD with 10 nminduced retardation from ITO substrate and with proper compensator.

[0038]FIG. 10 is a schematic view showing the basic components of amodulation optical system according to the preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

[0039] The present description is directed in particular to elementsforming part of, or cooperating more directly with, apparatus inaccordance with the invention. It is to be understood that elements notspecifically shown or described may take various forms well known tothose skilled in the art.

[0040] Referring to FIG. 1, there is shown in schematic form thearrangement of optical components in a digital projection apparatus 10,as described in commonly-assigned copending U.S. patent application Ser.No. 09/813,207, filed Mar. 20, 2001, entitled DIGITAL CINEMA PROJECTOR,by Kurtz et al., the disclosure of which is incorporated herein.Illumination optics 20 and pre-polarizer 45 precondition light from alight source 15 to provide illumination that is essentially uniformizedand polarized. Illumination optics 20 includes uniformizing optics, suchas an integrating bar or a fly's eye integrator assembly, and condensingrelay optics assembly. This light is subsequently polarized bypre-polarizer 45, with light of the desired polarization state directedtowards the polarization beamsplitter, while the rejected alternatepolarization state light nominally reflects back towards the lightsource. Pre-polarizer 45 is part of modulation optical system 40, whichalso comprises a wire grid polarization beamsplitter 50, a polarizationrotating spatial light modulator 55, and a polarization analyzer 60.Nominally, wire grid polarization beamsplitter 50 transmits the incidentlight having the preferred polarization state, while reflecting residualincident light having the alternate polarization state out of thesystem. Incident light is modulated by spatial light modulator 55, whichis nominally a liquid crystal display (LCD), to encode a two-dimensionalimage onto the light, which is then reflected as a modulated light beam.Wire grid polarization beamsplitter 50 reflects light from the modulatedlight beam having one polarization state, and transmits the light havingthe alternate polarization state. Projection optics 70 then directs thereflected modulated light beam onto a display surface 75, which isnominally a projection screen.

[0041] The design of digital projection apparatus 10 and modulationoptical system 40 both can be better understood from a deeper discussionof the properties of the wire grid polarizers used within these systems.FIG. 2 illustrates a basic prior art wire grid polarizer and definesterms that will be used in a series of illustrative examples of theprior art and the present invention. The wire grid polarizer 100 iscomprised of a multiplicity of parallel conductive electrodes (wires)110 supported by a dielectric substrate 120. This device ischaracterized by the grating spacing or pitch or period of theconductors, designated (p); the width of the individual conductors,designated (w); and the thickness of the conductors, designated (t).Nominally, a wire grid polarizer uses sub-wavelength structures, suchthat the pitch (p), conductor or wire width (w), and the conductor orwire thickness (t) are all less than the wavelength of incident light(λ). A beam of light 130 produced by a light source 132 is incident onthe polarizer at an angle θ from normal, with the plane of incidenceorthogonal to the conductive elements. The wire grid polarizer 100divides this beam into specular non-diffracted outgoing light beams;reflected light beam 140 and transmitted light beam 150. The definitionsfor S and P polarization used are that S polarized light is light withits polarization vector parallel to the conductive elements, while Ppolarized light has its polarization vector orthogonal to the conductiveelements. In general, a wire grid polarizer will reflect light with itselectric field vector parallel (“S” polarization) to the grid, andtransmit light with its electric field vector perpendicular (“P”polarization) to the grid. Wire grid polarizer 100 is a somewhat unusualpolarization device, in that it is an E-type polarizer in transmission(transmits the extraordinary ray) and O-type polarizer in reflection(reflects the ordinary ray).

[0042] When such a device is used at normal incidence (θ=0 degrees), thereflected light beam 140 is generally redirected towards the lightsource 132, and the device is referred to as a polarizer. However, whensuch a device is used at non-normal incidence (typically 30°<θ<60°), theilluminating beam of light 130, the reflected light beam 140, and thetransmitted light beam 150 follow distinct separable paths, and thedevice is referred to as a polarization beamsplitter. The detaileddesign of a wire grid device, relative to wire pitch (p), wire width(w), wire duty cycle (w/p), and wire thickness (t), may be optimizeddifferently for use as a polarizer or a polarization beamsplitter. Itshould be understood that both digital projection apparatus 10 andmodulation optical system 40, when designed with polarization modifyingspatial light modulators, may use polarization analyzers andpolarization beamsplitters other than wire grid type devices. Forexample, the polarization beamsplitter may be a MacNeille type glassprism, or the polarizer may be either a dye/polymer based sheetpolarizer. Within this discussion, the polarizing beamsplitter isassumed to be a wire grid type device, while both the pre-polarizer 45and analyzer 60 are also generally considered to be wire grid devices aswell, although that is not required for all configurations for theprojector.

[0043] The preferred spatial relationships of these polarizers, as usedin a modulation optical system 200, are illustrated in FIG. 3. The basicstructure and operation of modulation optical system 200 are describedin commonly-assigned copending U.S. patent application Ser. No.09/813,207, filed Mar. 20, 2001, entitled DIGITAL CINEMA PROJECTOR, byKurtz et al., the disclosure of which is incorporated herein. Modulationoptical system 200, which is a portion of an electronic projectionsystem, comprises an incoming illumination light beam 220, focusedthrough pre-polarizer 230, wire grid polarization beamsplitter 240, acompensator 260, and onto spatial light modulator 210 (the LCD) by acondenser 225. A modulated, image-bearing light beam 290 is reflectedfrom the surface of spatial light modulator 210, transmitted throughcompensator 260, reflected off the near surface of wire gridpolarization beamsplitter 240, and is subsequently transmitted through apolarization analyzer 270. After leaving modulation optical system 200,modulation image bearing light beam 290 follows along optical axis 275,and is transmitted through recombination prism 280 and projection lens285 on its way to the screen. Pre-polarizer 230 and polarizationanalyzer are assumed to both be wire grid polarization devices. A fullcolor projection system would employ one modulation optical system 200per color (red, green, and blue), with the color beams re-assembledthrough the recombination prism 280. Condensor 225, which will likelycomprise several lens elements, is part of a more extensive illuminationsystem which transforms the source light into a rectangularly shapedregion of nominally uniform light which nominally fills the active areaof spatial light modulator 210.

[0044] In a modulation optical system 200 utilizing a prior art wiregrid polarization beamsplitter, the wire grid polarization beamsplitter240 consists of a dielectric substrate 245 with sub-wavelength wires 250located on one surface (the scale of the wires is greatly exaggerated).Wire grid polarization beamsplitter 240 is disposed for reflection intoprojection lens system 285, thereby avoiding the astigmatism and comaaberrations induced by transmission through a tilted plate. Compensator260 is nominally a waveplate which provides a small amount of retardanceneeded to compensate for geometrical imperfections and birefringenceeffects which originate at the surface of spatial light modulator 210.For example, as discussed in U.S. Pat. No. 5,576,854 (Smith et al),compensator 260 may provide 0.02 λ's of retardance (A-plate) to correctfor polarization errors caused by residual geometrical imperfections ofthe LCD polarizing layer and residual thermally induced birefringencewithin the counter electrode substrate within the LCD package. In lessdemanding applications than digital cinema, compensator 260 may proveoptional.

[0045] The construction of modulation optical system 200, as used in adigital cinema application, is defined both by the system specificationsand the limitations of the available wire grid polarizing devices. Inparticular, digital cinema requires the electronic projector to providehigh frame sequential system contrast (1,000:1 or better). To accomplishthis, the polarization optical components, excluding spatial lightmodulator 210 (the LCD) of modulation optical system 200 must provide atotal optical system contrast (Cs) of ˜2,000:1. The actual targetcontrast for the polarization optics does depend on the performance ofthe LCDs. Thus, if for example, the LCDs provide only ˜1500:1 contrast,then the polarization optics must provide ˜3,000:1 contrast. Forexample, an LCD with vertically aligned molecules is preferred for thedigital cinema application due to its high innate contrast. Notably, thecontrast performance of both the LCDs and the polarization opticstypically decrease with increasing numerical aperture of the incidentbeam. Unfortunately, with today's technologies it is not sufficient touse just a single wire grid polarization beamsplitter 240 by itself inorder to meet the 2,000:1 target contrast for the polarization optics.For this reason, modulation optical system 200 also uses a wire gridpre-polarizer 230 and a wire grid polarization analyzer 270 to providethe target polarization performance.

[0046] The construction and operation of modulation optical system 200can be understood in yet greater detail, relative to its polarizationperformance. Preferably, pre-polarizer 230 is oriented to transmit “P”polarized light into the modulation optical system. Wire gridpolarization beamsplitter 240 is oriented with its sub-wavelength wirepattern oriented nominally parallel to the sub-wavelength wires ofpolarizer 230 (that is, the two devices are not crossed). Thus, thetransmitted “P” light is further modified (contrast enhanced) bytransmission through the wire grid polarization beamsplitter. Thetransmitted light beam then passes through compensator 260 andencounters spatial light modulator 210, which is nominally a reflectiveLCD, which modifies the polarization state of the incident light on apixel to pixel basis according to the applied control voltages.Intermediate code values, between white and black, reduce the amount of“On” state and increase the amount of “Off” state light. The “On” statelight, which has been polarization rotated, is in the “S” polarizationstate relative to the wire grid beamsplitter 240. Thus the “S” statelight reflects off the wire grid polarization beamsplitter 240, issubsequently transmitted through an optional compensator 265 (see FIGS.5a and 10) and polarization analyzer 270, and directed to the screen bya projection lens 285. The overall contrast (Cs) for modulation opticalsystem 200 (ignoring the LCD and compensator contributions) can beapproximated by:

1/Cs=1/(C _(T1) *C _(T2))+1/(C _(R2) *C _(T3))

[0047] where C_(T1) is the transmitted contrast of the wire gridpre-polarizer 230, C_(T2) and C_(R2) are transmitted and reflectedcontrast ratios for the wire grid polarization beamsplitter 240, andC_(T3) is the transmitted contrast for the wire grid polarizationanalyzer 270. In this system, the overall contrast is largely determinedby the low reflected contrast ratio C_(R2) for “S” polarization statelight off of wire grid polarization beamsplitter 240. The analyzercontrast C_(T3) needs to be quite high to compensate for the low C_(R2)values (˜30:1). Whereas the transmitted contrasts C_(T1) and C_(T2) donot need to be particularly high, provided that the respective contrastvalues are reasonably uniform over the spectrum. Polarization analyzer270 is oriented so that the “On” state light, which reflects off thewire grid polarization beamsplitter 240 and has “S” polarizationrelative to the wire grid polarization beamsplitter 240, sees this samelight as “P” state light relative to its own structure. Polarizationanalyzer 270 therefore removes any alternate polarization leakage lightaccompanying the desired “On” state beam.

[0048] As an example, in green light at 550 nm, wire grid pre-polarizer230 has an angle averaged polarization contrast ratio of ˜250:1. Whenused in combination, wire grid polarization beamsplitter 240 and wiregrid pre-polarizer 230 produce an on screen frame sequential opticalcontrast ratio of ˜25:1, which falls way short of the systemrequirements. Thus, the polarization performance of overall modulationoptical system 200 is also supported with the addition of wire gridpolarization analyzer 270, which is nominally assumed to be identical towire grid polarizer 230. Polarization analyzer 270 removes the leakageof light that is of other than the preferred polarization state,boosting the theoretical overall system contrast Cs to ˜2900:1.Performance does vary considerably across the visible spectrum, with thesame combination of wire grid polarizing devices providing ˜3400:1contrast in the red spectrum, but only ˜900:1 contrast in the blue.Certainly, this performance variation could be reduced with the use ofcolor band tuned devices, if they were available.

[0049] Modulation optical system 200 is best constructed with wire gridpolarization beamsplitter 240 oriented with the surface with thesub-wavelength wires 250 facing towards the spatial light modulator 210,rather than towards the illumination optics (condenser 225) and lightsource (see FIG. 3). While the overall contrast (Cs) is ˜2,900:1 whenthis orientation is used, the net contrast drops precipitously to ˜250:1when the alternate orientation (wires on the surface towards the lightsource) is used. This difference in overall contrast when modulationoptical system 200 is constructed with the image light reflecting offthe wire grid polarization beamsplitter 240, as a function of whetherthe sub-wavelength wires 250 face the spatial light modulator or thelight source, may be less important for slower (larger f#) systems.Additionally, referring to FIG. 3, modulation optical system 200provides the highest contrast and light efficiency when thesub-wavelength wires 250 of wire grid polarization beamsplitter 240 areoriented “vertically” (“into the page”, as shown), rather than“horizontally” (within the plane of the page). Wire grid polarizationbeamsplitter 240 can also be rotated (about the surface normal) by a fewdegrees to tune the contrast performance.

[0050] In order to build a digital cinema projector it is necessary tosimultaneously maximize luminance (10,000-15,000 lumens) and contrast(1,000:1+) with a system illuminating 35-55 ft. wide screens, whiledealing with the limitations of the various optics, wire grid devicesand LCDs. Luminance can be maximized by increasing the acceptance angle(numerical aperture) of light incident at the wire grid polarizationbeamsplitter and the LCD. With a wider acceptance angle (or a lower F#),the projection optics are able to gather more light. However, at thesame time, the wider the angle of source light incident at wire gridpolarization beamsplitter, the larger the leakage light from otherpolarization states and thus the smaller the contrast ratio (CR)available. Referring to FIG. 4, there is shown a graph of contrast formodulation optical system 200 (including wire grid pre-polarizer 230,wire grid polarization beamsplitter 240, a VA LCD, and a wire gridpolarization analyzer 270) vs. the F# of the light transmitted throughthe system. The plot of system contrast 300 shows that at approximatelyF/2.3, a contrast ratio of ˜600:1 is achieved. This value issignificantly less than the 1,000:1⁺ contrast needed for digital cinemaprojection. However, efficiency calculations suggest that an LCD baseddigital cinema projector will need to operate below F/3.0 to meet thescreen luminance targets, with systems speeds of F/2.0 to F/2.3 beingpotentially required for the larger screens.

[0051] Referring to FIG. 5a, there is shown a perspective viewrepresenting light polarization states for light reflected by andtransmitted through wire grid polarization beamsplitter 240 within amodulation optical system, for a pixel of LCD 210. A collimated orspecular pre-polarized beam 350 is transmitted through wire gridpolarization beamsplitter 240. As shown in FIG. 5a, the electric fieldpolarization of transmitted beam 355 is on a vector perpendicular to thewire grid of wire grid polarization beamsplitter 240. A returningmodulated beam 360 is reflected from the pixel on LCD 210, where the “S”polarized light is the image data, and the “P” polarized light is to berejected. Ideally, wire grid polarization beamsplitter 240 transmits100% of the unwanted “p” light as a modulated transmitted light 370.However, a small leakage light 365 is reflected from wire gridpolarization beamsplitter 240 and accompanies “s” modulated beam 360,causing reduced contrast (ratio of “s” to “p”). Relative to themodulated beam 360, wire grid beamsplitter acts as a pre-polarizer intransmission and a polarization analyzer in reflection, comprising thetypical crossed polarizer configuration.

[0052] While some loss of polarization contrast does occur with on axiscollimated light, the effects are more dramatic for oblique and skewrays. To better understand this, FIG. 5a includes an illustration of thebeam geometry for a large NA non-specular beam incident on a 45° tiltedsurface of wire grid polarization beamsplitter 240, while FIG. 5b showsthe geometry for a beam incident normal to a surface (such as the LCD210, pre-polarizer 230 or analyzer 270). For the normally incident case,the incoming beam is described by an azimuthal sweep of 0-180°, whilethe polar sweep of angles is limited (0-15° for F/2.0). The oblique raysare those rays that fall in the four quadrants outside the axes(azimuthal angles 0° and 180°, 90° and 270°) defined by the crossedpolarizers, and which lie in planes which contain the local optical axis275. The skew rays are the rays that lie in planes that do not containthe local optical axis 275. For the case of incidence to the 45° tiltedsurface, the incoming beam is again defined by an azimuthal sweep of0-180°, while the polar sweep of angles covers ˜0-15° relative to theoptical axis, or a sweep of ˜30-60° relative to the wire grid surface.This beam geometry will be important in appreciating the results givenby FIGS. 8a-i.

[0053]FIG. 6a illustrates the polarization contrast profile for crossedpolarizers, visible in angular space, and known as the “iron cross”. Theiron cross pattern 320 demonstrates peak extinction in directionsparallel and perpendicular to the grid of the analyzer, and diminishedextinction for the skew rays and oblique rays in the four off-axisquadrants. As the wire grid polarization beamsplitter has superiorangular performance when compared to most existing polarizers, thesedevices have been generally considered to not have a skew ray problem,and therefore to not require further polarization compensation. This isin part because the wire grid polarization beams splitter functions asan O-type polarizer in reflection and an E-type polarizer intransmission, and therefore is partially self compensating when used inboth transmission and reflection as in modulation optical system 200.However, even so, the extinction of the wire grid polarizationbeamsplitter is still not adequate for demanding applications likedigital cinema.

[0054] In the original electronic projection systems that were developedutilizing reflective liquid crystal displays, each LCD was addressedfrom behind using a CRT. Today, state of the art reflective LCDs aredirectly electronically addressed by means of a silicon backplane. Thesemodern devices, which are known as liquid crystal on silicon (LCOS)displays, generally comprise a silicon substrate, which is patternedwith pixel addressing circuitry, over coated with reflective and lightblocking layers, followed by an LCD alignment layer, a thin (˜5 μm)layer of liquid crystal, and an anti-reflection (AR) coated cover glass.The inside surface of the cover glass for a VA LCD has an ITO electrodeaddressing layer and an alignment layer on the internal surface,abutting the liquid crystal layer. The optical performance of an LCDdepends on many design parameters, including the material properties ofthe liquid crystals, the electrode structure, the pixel patterning andproximity, the ON state and OFF state orientations of the liquid crystalmolecules, the use and construction of the alignment layers, the opticalproperties of the reflective, anti-reflective, and light blockinglayers, etc. For example, while the liquid crystal molecules arenominally vertical to the inside surfaces of the silicon substrate andthe cover glass, in actuality the surface adjacent molecules areoriented with a residual tilt of 1-2 degrees from the normal. If thisresidual tilt angle becomes larger, device contrast starts to suffer.

[0055] The “iron cross” illustration of FIG. 6a also represents thenominal polarization response of an ideal VA LCD, as seen throughcrossed polarizers, assuming it has a negligible tilt angle. However,the net contrast provided by the modulation optical system can bedegraded by various subtle effects within either the LCDs (large tiltangles, bias voltages for the OFF state, thermally induced stresses, andlarge incident angles (large NA's)) or within the polarization devices,including the wire grid polarization beamsplitter (such as wire surfaceorientation, wire rotation, and large incident angles (large NA's).These effects can either cause the contrast to be generally reducedwhile the iron cross pattern 320 is retained, or cause the iron crosspattern 320 to be deformed into another extinction pattern (a “baseball”pattern 325 shown in FIG. 6b, for example). In the case of modulationoptical system 200, which partially comprises a wire grid pre-polarizer230, a wire grid polarization beamsplitter 240, a vertically aligned LCD210, and a wire grid polarization analyzer 270, the nominal system onlyprovides ˜600:1 contrast in the green at F/2.3, which is belowspecification. The system contrast can be enhanced, to meet and exceedspecification, through the use of the appropriate compensators.Certainly polarization contrast can be potentially enhanced by makingdesign changes to the actual polarization devices (the wire gridpolarization beamsplitter and the LCDs) themselves. However, as it isnot always possible or easy to alter the fundamental design,manufacturing, and performance limitations of these devices, alternatemethods of improving contrast have been sought. In particular, thecontrast performance of modulation optical system 200 has been enhancedwith new polarization compensators developed specifically to work withwire grid polarizers, and with new polarization compensators developedspecifically to work with the combination of vertically aligned LCDs andwire grid devices.

[0056] Compensators and polarizers are constructed from birefringentmaterials, which have multiple indices of refraction. Comparatively,isotropic media (such as glass) have a single index of refraction, anduniaxial media (such as liquid crystals) have two indices of refraction.Optical materials may have up to three principle indices of refraction.The materials with all three different refractive indices are calledbiaxial, and are uniquely specified by its principal indices nx₀, ny₀,nz₀, and three orientational angles as shown in FIG. 7a. FIG. 7b shows abiaxial film with the axes of nx₀, ny₀, and nz₀ aligned with x, y, and zaxes, respectively. The materials with two equal principal refractiveindices are called uni-axial materials. These two equal indices areordinary index and referred as n₀. The other different refractive indexis called an extraordinary index n_(e). The axis of n_(e) is alsoreferred to as an optical axis. Uniaxial materials are uniquelycharacterized by n_(e), n₀, and two angles describing the orientation ofits optical axis. When all three principal indices are equal, thematerials are called isotropic.

[0057] Light sees varying effective indices of refraction depending onthe polarization direction of its electric field when traveling througha uniaxial or biaxial material, consequentially, a phase difference isintroduced between two eigen-modes of the electric field. This phasedifference varies with the propagation direction of light, so thetransmission of the light varies with angle when uniaxial or biaxialmaterials are placed between two crossed polarizers. These phasedifferences translate into modifications of the local polarizationorientations for rays traveling along paths other than along or parallelto the optical axis. In particular, a compensator modifies or conditionsthe local polarization orientations for rays at large polar angles,which also includes both oblique and skew rays. A liquid crystalmaterial is typically a uniaxial material. When it is sandwiched betweentwo substrates as in a liquid crystal display, its optic axis generallychanges across the thickness depending on its anchoring at thesubstrates and the voltage applied across the thickness. A compensatoris constructed with one or more uniaxial and/or biaxial films, which aredesigned to introduce angularly dependent phase differences in a way tooffset the angle dependence of phase difference introduced by liquidcrystals or other optics. As is well known in the art, a uniaxial filmwith its optic axis parallel to the plane of the film is called aA-plate as shown in FIG. 7c, while a uniaxial film with its optic axisperpendicular to the plane of the film is called a C-plate, as shown inFIG. 7d. A uniaxial material with n_(e) greater than n₀ is called apositively birefringent. Likewise, a uniaxial material with n_(e)smaller than n₀ is called negatively birefringent. Both A-plates andC-plates can be positive or negative depending on their n_(e) and n₀. Amore sophisticated multi-layer compensator 400 has its optic axis orthree principal index axes varying across its thickness, as in FIG. 7e,where a stack of compensation films (birefringent layers 410 a, 410 b,and 410 c) are used with a substrate 420 to assemble the completecompensator. A detailed discussed of stack compensation can be found inU.S. Pat. No. 5,619,352 (Koch et al.). As is well known in art, C-platescan be fabricated by the use of uniaxially compressed polymers orcasting acetate cellulose, while A-plates can be made by stretchedpolymer films such as polyvinyl alcohol or polycarbonate.

[0058] The combination of crossed wire grid polarizers (wire gridpolarization beamsplitter 240, wire grid pre-polarizer 230, and wiregrid polarization analyzer 270) in modulation optical system provides anexcellent dark state for light traveling in the planes parallel orperpendicular to the wires. However, a maximum amount of light leakageoccurs when light travels at a large polar angle (theta) away from thepolarizer normal direction and 45/135 degree relative to the wires (FIG.5b shows the polar and azimuthal geometry for the polarizers). Forexample, with reference to FIG. 6a, for the standard “iron cross” typeextinction pattern, peak contrast along the axes can exceed 1,000:1,while contrast in the four quadrants located 45 degrees off the crossedcoordinate axes falls off to 300:1 or less. Light transiting theseangular regions, which includes skew rays, experiences less extinctionthan light closer to the axes. This loss of contrast from the quadrantrays and the skew rays can be significant for digital cinema projection,which again requires high optical system contrast (>2,000:1) and fastoptics (<F/3.0).

[0059] Wire grid polarizers have been studied by the use of effectivemedium theory (“Generalized model for wire grid polarizers”, Yeh, SPIEVol. 307, (1981), pp. 13-21). When the grating pitch (p) is much smallerthan the wavelength (λ), the subwavelength grating can be approximatelyconsidered as an uni-axial film with effective refractive indices.Although effective medium theory is much easier to be implemented andprovides a qualitative understanding of wire grid polarizers, itgenerally fails to obtain accurate results. It is especially true forcalculation of very low transmission through crossed wire gridpolarizers. The limitation of effective medium theory has been pointedout by Kihuta et al. (“Ability and limitation of effective medium theoryfor subwavelength gratings”, Optical Review 2, (1995) pp.92-99). As aresult, the wire grid polarizers have been modeled using the moreexacting rigorous coupled wave analysis (RCWA) discussed in Kuta et al.(“Coupled-wave analysis of lamellar metal transmission gratings for thevisible and the infrared”, Kuta, et al., Journal of the Optical Societyof America A, Vol. 12, (1995), pp.1118-1127). The results given in FIGS.8a through 8 i for wire grid polarizers are modeled using RCWA.

[0060]FIG. 8a shows the theoretical transmission through crossed wiregrid polarizers about normal incidence, and shows that the transmissionat a polar angle of 20 deg. (F/1.5) and an azimuthal angle of 45 deg. is0.99×10⁻³, which is 2.5× larger than the transmission of 0.4×10⁻³ at apolar angle of 0 deg. For an even larger polar angle, such as 40 deg.(F/0.8), at an azimuthal angle of 45 deg., the transmission loss is muchgreater, with the value of 5×10⁻³. The increased transmission translatesinto additional light leakage, and thus loss of contrast. For thesecalculations, the wire grid polarizers were modeled as aluminum wirestructures, deposited on Corning glass 1737F, with a wire pitch of 144nm (˜λ/4), a wire duty cycle of 0.45, and a wire height of 130 nm. Thewire grid polarizer is modeled in the green at 550 nm, with therefractive index of A1 being 0.974+i6.73, and the refractive index ofCorning glass is 1.52. These parameters are used for FIG. 8a throughFIG. 8i unless specified otherwise. As can be seen in FIG. 8a, themaximum light leakage (reduced contrast) occurs at 45 degrees relativeto the wire grid. FIG. 8a can be understood with reference to thegeometry of FIG. 5b, which shows that for the normally incident beam,the relevant cone of light is described by an azimuthal sweep of 0-180°and a polar sweep of ˜0-20° (F/1.5). The plot of FIG. 8a showsvariations in transmission for crossed polarizers vs. azimuthal andpolar angles, rather than the variations in contrast. Polarizationcontrast can be difficult to model in a comprehensive way for a complexsystem like modulation optical system 200. However, contrast isapproximately inversely proportional to the transmission for crossedpolarizers, such that small changes in transmitted light can cause hugechanges in system contrast. While FIGS. 8a-f also show significantoff-angle transmission effects for slower beams (in particular at 10°,or F/2.9), the data will be consistently presented at 20° (F/1.5) for amore dramatic comparison.

[0061] Notably, the general behavior of crossed polarizers to sufferlight leakage for oblique and skews rays at large polar angles does notchange substantially just by using better polarizers. For example,modeling has shown that even if the pitch of wire grid is much smallerthan the wavelength of the light, such as {fraction (1/100)}, asignificant amount of light still leaks through two crossed wire gridpolarizers at large polar angles. FIG. 8b shows the transmission throughcrossed wire grid polarizers, where the pitch of the wire gridpolarizers is 5.5 nm (λ/100). Certainly the fine pitch λ/100 device doesshow lower transmission than does the λ/4 device (0.23×10⁻³ vs. 0.4×10⁻³at a polar angle of 0 deg.), and thus provides higher contrast (thetheoretical contrast differences are much greater than 2× between λ100and λ/4 devices). In this case, the modeled λ/100 device shows increasedtransmission (and thus light leakage) at a polar angle of 20 deg. and anazimuthal angle of 45 deg. of 0.95×10⁻³, which is ˜4× larger than thetransmission of 0.23×10⁻³ at a polar angle of 0 deg. At a polar angle of40 degrees (and an azimuthal of 45 degrees), the transmission (lightleakage) is 10× greater (9.7×10⁻³). Thus, even for these λ/100 wiregrids, which are far finer than what is presently manufacturable, theoff axis behavior is largely the same, although the theoreticalextinction is greater.

[0062] Wire grid polarizers, which transmit the P-polarization as anextraordinary ray (E-type) and reflect the S-polarization as an ordinaryray (O-type), while only absorbing ˜10% of the incident light, cannot beaccurately treated as a uniaxial film. By comparison, the standard sheetpolarizer, which is manufactured by Polaroid Corporation, is similar tothe wire grid polarizer in that it uses “wires” (iodine atoms imbeddedin stretched PVA plastic), is actually a significantly different device.First, the sub-wavelength “wires” (p<<λ) of the dye sheet polarizer aresignificantly smaller than wires of the visible wavelength wire gridpolarizer (p˜/λ4). Moreover, the dye sheet polarizer is an O-typepolarizer, which transmits the ordinary wave and absorbs (rather thanreflects) the extraordinary wave. The standard dye sheet polarizer canbe accurately modeled as a uniaxial film with an extraordinary index andan ordinary index. Optiva Inc. recently developed an E-type sheetpolarizers based on supra-molecular lyotropic liquid crystallinematerial, which transmit the extraordinary wave and absorb the ordinarywave of incident light. (see Lazarev et al., “Low-leakage off-angle inE-polarizers”, Journal of the SID, vol. 9, (2001), pp. 101-105). TheOptiva polarizer is a sheet polarizer similar to the standard dye sheetpolarizer, except that it is an E-type polarizer which transmits theextraordinary wave and absorbs (rather than reflects) the ordinary wave.

[0063] When two standard O-type dye sheet Polaroid polarizers are usedin the crossed configuration, an iron cross pattern 320 (see FIG. 6a) isalso experienced. Light leaks through these conventional crossed sheetpolarizers at obliquely incident angles with maximum leakage occurringat 45 degrees relative to the transmission or absorption axes of thesheet polarizers. Various compensators have been proposed to reducelight leakage through crossed O-type polarizers, as published by Chen etal. and in Uchida et al. (T. Ishinabe, T. Miyahita, and T. Uchida,“Novel Wide Viewing Angle Polarizer with High Achromaticity”, SID 2000Digest, pgs. 1094-1097). According to Chen, a combination of uniaxialmaterials, an A-plate and a C-plate, dramatically reduces light leakageat off angle. One of its design requirements is that the optical axis ofthe A-plate should be parallel to the transmission axis of the adjacentpolarizer. Uchida solves the same problem using two biaxial films toconstruct the compensator.

[0064] Although wire grid polarizers (E-type polarizer in transmission,O-type in reflection) and standard sheet polarizers (O-type intransmission, E-type absorption) are significantly different devices,benefit might be obtained by combining an existing compensator for asheet polarizer with crossed wire grid polarizers. FIG. 8c shows thetransmission through crossed wire grid polarizers paired with a priorart sheet polarizer compensator from Chen at al., which consists of a137 nm A-plate and a 80 nm C-plate. For the case of a light beam at apolar angle of 20 deg. (F/1.5) and an azimuthal angle of 45 degrees,this compensator does provides significant improvement, reducing thetransmission to 0.52×10⁻³, which represents about 30% more light leakagethan the on-axis case for an uncompensated crossed polarizers(0.4×10⁻³). However, at greater polar angles, such as 40 degrees (again,an azimuthal angle of 45 degrees) this compensator still allowssubstantially greater transmission, at a level of 2.4×10⁻³, or ˜6×greater than the on-axis case. The pitch of the wire grid is againassumed to be 144 nm. Thus, while prior art sheet polarizer compensatorscan be used in combination with crossed wire grid polarizers to providesome polarization contrast improvement, there is yet room for furtherimprovement.

[0065] Fortunately, it is possible to design compensators which arespecifically optimized to work with wire grid polarizers and wire gridpolarization beamsplitters, and which can be used to boost the contrastprovided by modulation optical system 200. When wire grid polarizers areutilized as a polarizing beamsplitter, they first transmit light andthen reflect light, or first reflect light and then transmit light. Theangle at which light strikes the wire grid polarizer at the first timeis generally different from the angle at which light does at the secondtime. The new compensators have been developed to minimizing lightleakage through crossed wire grid polarizers at off angles withinmodulation optical system 200. Likewise, compensators have beendeveloped which reduce light leakage through a wire grid polarizingbeamsplitter.

[0066] As a first example, a polarization compensator was designed as acombination of an A-plate and C-plate, neither of which will affect theon-axis transmission while reducing the off-axis transmission. Thedesigned compensator, which enhances the performance of crossed wiregrid polarizers (wire grid pre-polarizer 230 and wire grid polarizationanalyzer 270 of FIG. 3), uses a combination of two specific birefringentfilms, a +275 nm A-plate and a −60 nm C-plate. FIG. 8d, which shows thetotal transmission through the combination of the crossed wire gridpolarizers and this first example compensator, shows a broad change inthe transmission response curves, indicating significant transmittedlight reductions as compared to FIG. 8a. The transmission is below0.48×10⁻³ for all polar angles up to 40 deg. (˜0.4×10⁻ at 20° polarangle), which is basically equal to the on-axis transmission for theuncompensated crossed wire grid polarizers (0.4×10⁻³). In actuality, thecompensator modifies or conditions the polarization orientations of theoblique and skew rays to improve their transmission through the crossedpolarizers, thereby enhancing the contrast of the modulated beam. Thisoptimized compensator for wire grid polarizers also providessignificantly better performance than does the sheet polarizercompensator discussed previously. Notably, the optical axis of theA-plate for this wire grid polarizer compensator is perpendicular to thetransmission axis of the adjacent polarizer. Whereas, by comparison, theprior art sheet polarizer compensator, as described by Chen et al.,requires that the optical axis of the A-plate to be parallel to thetransmission axis of the adjacent polarizer.

[0067] Although, this first example compensator design has significantlyimproved the performance of a modulation optical system 200 which usescrossed wire grid polarizers, where these wire grid devices have arelatively large pitch (p =144 nm ˜λ/4), the same compensator design canimprove the performance when wire grid devices with a smaller pitchesare used. For example, FIG. 8e shows the modeled performance of a finepitch device (p=5.5 nm ˜λ/100) with compensation, where the transmissionat a polar angle of 40 deg. and an azimuthal angle of 45 deg. hasdropped to 0.24×10⁻³ as compared to the prior uncompensated result of9.7×10⁻³ shown in FIG. 8b.

[0068] A second example compensator was designed for use with crossedwire grid polarizers, which also has a combination of an A-plate and aC-plate. In this case, the A-plate and C-plate both have positivebirefringence, with retardations of 137 nm and 160 nm, respectively.Unlike the first example compensator, the optical axis of the A-platefor this compensator is parallel to the transmission axis of theadjacent polarizer. FIG. 8f shows the improved transmission resultingfrom this compensator design, which is below 0.46×10⁻³ for all polarangles up to 20 degrees. However, at a polar angle of 40 deg. and anazimuthal angle of 45 deg. the transmission is only reduced to 1.1×10⁻³.While this design is not as good as the first example compensatordesign, particularly above 20 degree polar angle (see FIG. 8d), thelight leakage is still significantly reduced as compared to theuncompensated crossed polarizers (see FIG. 8a). As before, thiscompensator can be inserted into a modified the modulation opticalsystem 200 of FIG. 10, as an added element, secondary compensator 265.

[0069] In FIG. 10, which shows modified modulation optical system 200,the compensator used to optimize performance through the crossed wiregrid polarizers (pre-polarizer 230 and analyzer 270) is located prior toanalyzer 270, and is shown as secondary compensator 265. This samecompensator could alternately be located just after wire gridpre-polarizer 230, as indicated by alternate secondary compensator 266of FIG. 10. As another alternative, part of a designed compensator forthese crossed polarizers can be positioned as secondary compensator 265,while another portion is simultaneously provided as alternate secondarycompensator 266. That is an unlikely scenario, as both the componentcount and mounting requirements are increased. It is also a requirementthat the secondary compensator(s) 265 (and/or 266) be located in theoptical path between wire grid pre-polarizer 230 and wire gridpolarization analyzer 270. That means that secondary compensator 265cannot, for example, be located after wire grid polarization analyzer270.

[0070] Secondary compensator 265 can also be used in an unfolded opticalsystem without a polarization beamsplitter, as shown in FIG. 5c. In thiscase, transmitted polarized light exits wire grid pre-polarizer 230,passes through a spatial light modulator 210 (which is nominally atransmissive LCD), secondary compensator 265, and wire grid polarizationanalyzer 270. Alternately, the wire grid polarizer secondary compensator265 can be located prior to the spatial light modulator 210 withinmodulation optical system 200. As shown in FIG. 5c, the wire gridpre-polarizer 230 and wire grid polarization analyzer 270 are crossed,so that modulation optical system 200 is nominally in the Off state, andspatial light modulator 210 rotates light to transmit through the wiregrid polarization analyzer 270 to provide On state light. It should beunderstood that the wire grid pre-polarizer 230 and wire gridpolarization analyzer 270 can be aligned for nominal open statetransmission (not crossed), with the spatial light modulator 210rotating light for the Off state.

[0071] Polarization response improvement can also be provided for thewire grid polarization beamsplitters, as well as for the wire gridpolarizers. FIG. 8 g shows the combined transmission (product of thetransmitted light and reflected light) through wire grid polarizingbeamsplitters without polarization compensation, assuming that thespatial light modulator 210 is replaced with a perfect mirror. In thiscase, the incoming beam is incident on 45° tilted surface with a conedescribed by an azimuthal sweep of 0-180°, and a polar sweep of anglesof ˜0-40° (see FIG. 5a), where the light falls within 0-15° polar anglefor an F/2.0 beam. For example, FIG. 8g. shows a combined transmissionwithout polarization compensation of 6.5×10⁻² at a polar angle of 30°and an azimuthal angle of 45°.

[0072] A third example compensator was designed, in this case to enhancethe contrast provided by wire grid polarizing beamsplitter 240, as usedin the modulation optical system 200 of FIG. 10 along with spatial lightmodulator 210 (VA LCD). This compensator example has a combination of anA-plate and a C-plate, having retardations of 90 nm and 320 nm (bothwith positive birefringence), respectively. Within the layered structureof the compensator, the A-plate is preferentially located closer to thewire grid polarization beamsplitter than the C-plate, which is closer tothe LCD. The optical axis of A-plate is parallel to the transmissionaxis of the adjacent polarizer (perpendicular to the wires). FIG. 8hshows the combined transmission through a wire grid polarizingbeamsplitter used in combination with this compensator is reduced to2.7×10⁻² compared to 6.5×10⁻² at a polar angle of 30 degrees in FIG. 8g.Even at smaller polar angles, such as 15 or 20 degrees, the compensatorreduces transmission (less leakage) by ˜2× as compared to theuncompensated wire grid polarization beamsplitter. This compensator isshown in the modified modulation optical system 200 of FIG. 10 ascompensator 260, and is located between wire grid polarizationbeamsplitter 240 and liquid crystal spatial light modulator 210. This isthe only acceptable location for this compensator within modulationoptical system 200.

[0073] A fourth example compensator was designed, as with the lastexemplary device, to enhance the combined transmission provided by wiregrid polarizing beamsplitter 240 used in the modulation optical system200 of FIG. 10 along with spatial light modulator 210 (VA LCD). Thiscompensator is a combination of A-plate and C-plate having a retardationof 90 nm and −200 nm, respectively (positive and negativebirefringence). The compensator of FIG. 8i provides a smaller combinedtransmission, which is 3.5×10⁻² compared to 6.5×10 ⁻² in FIG. 8g. Unlikethe third example compensator, the optical axis of the A-plate for thiscompensator is perpendicular to the transmission axis of the adjacentpolarizer (parallel to the wires), rather than parallel to thetransmission axis (perpendicular to the wires). As before, thiscompensator is shown in the modified modulation optical system 200 ofFIG. 10 as compensator 260.

[0074] It should be emphasized that the prior art does not describe howto design a modulation optical system for a projection display usingboth LCDs and wire grid devices, which further has polarizationcompensators to boost contrast. Certainly, the actual exemplarycompensators designed for use with the wire grid devices can haveconventional structures and combinations of materials (such aspolycarbonate or acetate) as have been previously described for otherpolarization devices. However, wire grid polarizers are distinctlydifferent from the prior art devices (sheet polarizers and MacNeilleprisms for example) in subtle and non-obvious ways, and therefore thedesign of the associated optimized compensators cannot be easilyextrapolated from the prior compensator designs.

[0075] It is of course understood that various designs can achievecomparable performances as described above or even better. It is alsounderstood that a single biaxial film can be used to replace thecombination of A-plate and C-plate for any of these exemplarycompensators. It should also be understood that the modeled compensatorscan be designed in reverse order, with the C-plate encountered beforethe A-plate, rather than the order of A-plate and then C-plate providedin the above examples. When the order is switched, the designedbirefringence values likely change. It is also understood thatadditional A-plate and/or C-plate and/or biaxial films can be added tothe combination of A-plate and C-plate for any of these exemplarycompensators.

[0076] Certainly, as with the addition of any other optical componentinto a system, the usual concerns for providing the mounting and ARcoatings for these compensators also apply. The compensators may beconstructed with their birefringent films sandwiched between two glasssubstrates, with optical matching adhesives or gels holding the elementstogether. In that case, any glass to air surfaces should be AR coated.Alternately, the compensators can be integrated with the wire gridpolarizers (wire grid pre-polarizer 230 and wire grid polarizationanalyzer 270) and mounted directly to the glass substrates of thesecomponents. That reduces the part count, the count of glass to airsurface interactions, and the mounting issues. However, the compensatorshould be mounted to the flat glass surface of the wire grid device, andnot to the surface with the wire grid coating.

[0077] Although the above examples are designed for a single wavelengthat 550 nm, it should be understood that these examples function for allother wavelengths equally well as for 550 nm provided that the materialof the compensator has a dispersion matched with wavelength. This meansthat ratio of the retardation/wavelength is substantially unchangedacross all visible wavelengths.

[0078] It should also be understood that modulation optical system 200can be constructed in a variety of combinations. As depicted in FIG. 10,it includes wire grid pre-polarizer 230, wire grid polarizationbeamsplitter 240, wire grid polarization analyzer 270, compensator 260,secondary compensator 265, and alternate secondary compensator 266.However the system could be constructed with wire grid pre-polarizer230, wire grid polarization beamsplitter 240, wire grid polarizationanalyzer 270, and compensator 260, with the secondary compensators leftout. Likewise, the system could be constructed with wire gridpolarization beamsplitter 240 and compensator 260, but with wire gridpre-polarizer 230 and wire grid polarization analyzer 270 as non-wiregrid devices, and with the secondary compensators left out. Alternatelyagain, the system could be constructed with wire grid pre-polarizer 230,wire grid polarization beamsplitter 240, wire grid polarization analyzer270, and secondary compensator 265, but with compensator 260 left out.Needless to say, yet other combinations of components are possible.

[0079] The overall contrast performance of modulation optical system 200of FIG. 10 can be enhanced not only be providing compensators whichoptimize the performance of the crossed wire grid polarizers or the wiregrid polarization beamsplitter, but also which enhance the performanceof the LCDs as seen through the wire grid polarization beamsplitter. Bycomparison, in the prior art, U.S. Pat. No. 5,576,854 (Schmidt et al.) acompensator is described which optimizes for the VA LCD working incombination with a MacNeille beamsplitter. As disclosed in U.S. Pat. No.5,576,854, a 0.27λ compensator is used, where 0.25 λ's compensate forthe MacNeille prism and 0.02λ's for birefringence in the counterelectrode substrate. The counter electrode substrate is susceptible tothermal gradients that cause stresses within the substrate, which inturn cause localized birefringence. Even with carefully chosen materialsfor the substrate glass, such as SF-57 or fused silica, a smallretardance, such as 0.02λ's was used to compensate for residual lightleakages in the dark state from stress birefringence. When a verticalaligned LCD is in a non-active state without any voltage applied, thelight leakage at on axis is small. However, in practice the dark stateis a state with a non-zero voltage called as Voff. This voltage causesthe liquid crystal to tilt down, and can significantly increases lightleakage. Compensators have also been designed by others for example,U.S. Pat. No. 5,298,199 (Hirose, et al.) to correct for this effect.

[0080] In the case of a vertically aligned LCD combined with a wire gridpolarizing beamsplitter, the 0.25λ's retardance used in U.S. Pat. No.5,576,854 for the MacNeille type prism is not required. However, theresidual 0.02λ's retardance (˜11 nm), which is provided as an A-plate,may still be useful to correct to stress birefringence within the VALCD, even with wire grid devices. In addition, a compensator optimizedfor a VA LCD may also include a negative C-plate when used in fastoptical systems, including a digital cinema system operating at F/3.0 orbelow. Thus, preferred compensators for reflective VA LCD's used incombination with wire grid polarizers comprise a negative C-plate and apositive A-plate. The negative C-plate is preferred to have same amountof retardation as the liquid crystal (+233 nm for example), but withopposite sign, to correct the viewing angle dependence of the liquidcrystal. This viewing angle dependent retardation present in the liquidcrystal is typically ˜160-250 nm.

[0081] As an example, FIG. 9a shows the contrast contour plot for lightreflected off of an ideal VA LCD through crossed polarizers in the Offstate. This corresponds to the “iron cross” pattern 320 of FIG. 6a, withminimal light along the optical axis (center of the spherical pattern)and along the directions parallel or perpendicular to the transmissionaxis of the crossed polarizers. However, as the iron cross pattern 320shows, some leakage light can be expected in the four quadrants. FIG. 9bshows contrast contour plot for light reflected off a VA LCD with 10 nmresidual retardation from induced birefringence in the substrate, whichcorresponds to the baseball pattern 325 of FIG. 6b. Unfortunately, whenthis baseball pattern 325 occurs, leakage light into the projectionsystem is significantly increased, and the contrast is reduced. FIG. 9cshows contrast contour plot for a VA LCD with proper compensators (−233nm C-plate) designed according to the present invention inserted at theabove discussed locations. The 1000:1 iso-contrast curve extends to morethan 13° of polar angle. This compensator can be inserted into modifiedoptical modulation system 200 of FIG. 10, immediately prior to the LCD210, as compensator 260.

[0082] In actuality, the compensators for the wire grid polarizationbeamsplitter 240 and the LCD 210 are co-located between these twocomponents, and can be combined into one packaged compensator device.The exemplary compensator for the wire grid polarization beamsplitter240 corresponding to FIG. 8h used a combination of an A-plate and aC-plate having a retardation of 90 nm and 320 nm. By comparison, thevertically aligned LCD has a retardation of ˜233 nm, and requires aC-plate with a −233 nm retardation for correction. When these twoC-plate designs are combined, the remaining C-plate has only ˜87 nmretardance. Thus, the cancellation between the compensator and the LCDsignificantly decreases the amount of additional retardance needed. Thecombined compensator 260 then comprises the 11 nm A-plate for the VA LCD(0.02λ's compensation), the 87 nm C-plate, and the 90 nm A-plate for thewire grid polarization beamsplitter 240 in sequential order, with the 11nm A-plate located closest to the LCD 210. The two A-plates cannot besimply combined, as the 11 nm A-plate needs to be rotatable, while the90 nm A-plate has a fixed orientation relative to the sub-wavelengthwires 250. Thus, what is provided is an apparatus and method forachieving high levels of contrast by using a wire grid polarizationbeamsplitter with a compensator for minimizing leakage light in thepixel black (OFF) state for a VA LCD.

[0083]FIG. 4 shows a graph 310 that relates system contrast to therelative F# for a modulation optical system comprising a VA LCD, wiregrid polarizers, a wire grid polarization beamsplitter, and acompensator, which correct for the unwanted P polarization in returningmodulated beam. In this case, a customized version of compensator 260 isused. Notably, although use of a compensator can actually reduce CR athigher F# values, the compensator improves contrast at low values, belowapproximately F/4.0. Contrast 310 may not always be better, becausecompensators can be complex structures, which suffer undesiredreflections and defects.

[0084] It should be understood that the polarization compensationconcepts developed within this application for optimizing thepolarization performance of wire grid polarizer devices could be used inmodulation optical systems which have spatial light modulators otherthan vertically aligned LCDs. For example, spatial light modulator 210could also be a 60 degree twisted nematic LCD, a PLZT modulator, or someother polarization rotating modulator.

[0085] The invention has been described in detail with particularreference to certain preferred embodiments thereof, but it will beunderstood that variations and modifications can be effected within thescope of the invention as described above, and as noted in the appendedclaims, by a person of ordinary skill in the art without departing fromthe scope of the invention.

PARTS LIST

[0086]10. Digital projection apparatus

[0087]15. Light source

[0088]20. Illumination optics

[0089]40. Modulation optical system

[0090]45. Pre-polarizer

[0091]50. Wire grid polarization beamsplitter

[0092]55. Spatial light modulator

[0093]60. Parization analyzer

[0094]70. Projection optics

[0095]75. Display surface

[0096]100. Wire grid polarizer

[0097]110. Conductive electrodes or wires

[0098]120. Dielectric substrate

[0099]130. Beam of light

[0100]132. Light Source

[0101]140. Reflected light beam

[0102]150. Transmitted light beam

[0103]200. Modulation optical system

[0104]210. Spatial light modulator (LCD)

[0105]220. Illumination light beam

[0106]225. Condensor

[0107]230. Wire grid pre-polarizer

[0108]240. Wire grid polarization beamsplitter

[0109]245. Dielectric substrate

[0110]250. Sub-wavelength wires

[0111]260. Compensator

[0112]265. Secondary compensator

[0113]266. Alternate secondary compensator

[0114]270. Wire grid polarization analyzer

[0115]275. Optical axis

[0116]285. Projection lens

[0117]280. Recombination prism

[0118]290. Modulated image-bearing light beam

[0119]300. System contrast

[0120]310. Graph

[0121]320. Iron Cross pattern

[0122]325. Baseball pattern

[0123]350. Pre-polarized beam

[0124]355. Transmitted beam

[0125]360. Modulated beam

[0126]365. Leakage light

[0127]370. Transmitted light

[0128]400. Multi-layer compensator

[0129]410 a. Birefringent layers

[0130]410 b. Birefringent layers

[0131]410 c. Birefringent layers

[0132]420. Substrate

What is claimed is:
 1. A display apparatus comprising: (a) a lightsource for forming a beam of light; (b) a pre-polarizer for polarizingsaid beam of light to provide a polarized beam of light; (c) a wire gridpolarizing beamsplitter for receiving said polarized beam of light, fortransmitting said polarized beam of light having a first polarization,and for reflecting said polarized beam of light having a secondpolarization; (d) a reflective liquid crystal device for selectivelymodulating said polarized beam of light having a second polarization toencode image data thereon in order to form a modulated beam, and forreflecting said modulated beam back to said wire grid polarizingbeamsplitter; (e) a compensator, located between said wire gridpolarization beamsplitter and said reflective liquid crystal device, forconditioning oblique and skew rays of said modulated beam; (f) whereinsaid wire grid polarizing beamsplitter reflects said compensatedmodulated beam; (g) a polarization analyzer which removes residualunmodulated first polarization light; and (h) image-forming optics forforming an image from said modulated beam.
 2. The apparatus of claim 1wherein said compensator comprises one or more birefringent layers,wherein said birefringent layers comprise at least one of the following;an A-plate film, a C-plate film, or a biaxial film.
 3. The A-plateaccording to claim 2 wherein the optical axis of said A-plate issubstantially parallel to the sub-wavelength wires of said wire gridpolarization beamsplitter.
 4. The A-plate according to claim 2 whereinthe optical axis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization beamsplitter.
 5. Theapparatus of claim 1 wherein said reflective liquid crystal device has avertically aligned construction.
 6. The compensator of claim 1 whichmodifies polarization states of the oblique and skew rays relative tosaid wire grid polarization beamsplitter, or said reflective liquidcrystal device, or both.
 7. A modulation optical system for providinghigh contrast modulation of an incident light beam, comprising: (a) apre-polarizer for pre-polarizing said beam of light to provide apolarized beam of light; (b) a wire grid polarization beamsplitter forreceiving said polarized beam of light, for transmitting said polarizedbeam of light having a first polarization, and for reflecting saidpolarized beam of light having a second polarization; (c) a reflectiveliquid crystal device for selectively modulating said polarized beam oflight having a first polarization to encode image data thereon in orderto form a modulated beam, and for reflecting said modulated beam back tosaid wire grid polarizing beamsplitter, (d) a compensator, locatedbetween said wire grid polarization beamsplitter and said reflectiveliquid crystal device, for conditioning oblique and skew rays of saidmodulated beam; (e) a polarization analyzer which removes residualunmodulated first polarization light.
 8. The system of claim 7 whereinsaid compensator comprises one or more birefringent layers, wherein saidbirefringent layers comprise at least one of the following; an A-platefilm, a C-plate film, or a biaxial film.
 9. The A-plate according toclaim 8 wherein the optical axis of said A-plate is substantiallyparallel to the sub-wavelength wires of said wire grid polarizationbeamsplitter.
 10. The A-plate according to claim 8 wherein the opticalaxis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization beamsplitter. 11.The system of claim 7 wherein said reflective liquid crystal device hasa vertically aligned construction.
 12. The compensator of claim 7 whichmodifies polarization states of the oblique and skew rays relative tosaid wire grid polarization beamsplitter, or said reflective liquidcrystal device, or both.
 13. A modulation optical system for providinghigh contrast modulation of an incident light beam, comprising: (a) awire grid pre-polarizer for pre-polarizing said beam of light; (b) awire grid polarization beamsplitter for receiving said polarized beam oflight, for transmitting said polarized beam of light having a firstpolarization, and for reflecting said polarized beam of light having asecond polarization; (c) a reflective liquid crystal device forselectively modulating said polarized beam of light having a firstpolarization to encode image data thereon in order to form a modulatedbeam, and for reflecting said modulated beam back to said wire gridpolarizing beamsplitter; (d) a compensator, located between said wiregrid polarization beamsplitter and said reflective liquid crystaldevice, for conditioning oblique and skew rays of said modulated beam;and (e) a wire grid polarization analyzer which removes residualunmodulated first polarization light.
 14. The system of claim 13 whereinsaid compensator comprises one or more birefringent layers, wherein saidbirefringent layers comprise at least one of the following; an A-platefilm, a C-plate film, or a biaxial film.
 15. The A-plate according toclaim 14 wherein the optical axis of said A-plate is substantiallyparallel to the sub-wavelength wires of said wire grid polarizationbeamsplitter.
 16. The A-plate according to claim 14 wherein the opticalaxis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization beamsplitter. 17.The system of claim 13 wherein said reflective liquid crystal device hasa vertically aligned construction.
 18. The compensator of claim 13 whichmodifies polarization states of the oblique and skew rays relative tosaid wire grid polarization beamsplitter, or said reflective liquidcrystal device, or both.
 19. A modulation optical system for providinghigh contrast modulation of an incident light beam, comprising: (a) awire grid pre-polarizer for pre-polarizing said beam of light; (b) awire grid polarization beamsplitter for receiving said polarized beam oflight, for transmitting said polarized beam of light having a firstpolarization, and for reflecting said polarized beam of light having asecond polarization; (c) a reflective liquid crystal device forselectively modulating said polarized beam of light having a firstpolarization to encode image data thereon in order to form a modulatedbeam, and for reflecting said modulated beam back to said wire gridpolarizing beamsplitter; (d) a wire grid polarization analyzer whichremoves unmodulated first polarization light; and (e) a compensatorwhich conditions oblique and skew rays relative to said wire-gridpolarization analyzer and said wire grid pre-polarizer.
 20. The systemof claim 19 wherein said compensator comprises one or more birefringentlayers, wherein said birefringent layers comprise at least one of thefollowing; an A-plate film, a C-plate film, or a biaxial film.
 21. TheA-plate according to claim 20 wherein the optical axis of said A-plateis substantially parallel to the sub-wavelength wires of said wire gridpolarization analyzer.
 22. The A-plate according to claim 20 wherein theoptical axis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization analyzer.
 23. Thesystem of claim 19 wherein said reflective liquid crystal device has avertically aligned construction.
 24. The system of claim 19 wherein saidcompensator is located between said wire grid pre-polarizer and saidwire grid polarization beamsplitter.
 25. The system of claim 19 whereinsaid compensator is located between said wire grid polarizationbeamsplitter and said wire grid polarization analyzer.
 26. A modulationoptical system for providing high contrast modulation of an incidentlight beam, comprising: (a) a wire grid pre-polarizer for pre-polarizingsaid beam of light; (b) a transmissive liquid crystal device forselectively modulating said polarized beam of light having to encodeimage data thereon in order to form a modulated beam; (c) a wire gridpolarization analyzer which transmits said modulated beam and blocks andunmodulated beam; and (d) a compensator located between saidtransmissive liquid crystal device and said wire grid polarizationanalyzer which conditions oblique and skew rays.
 27. The system of claim26 wherein said compensator comprises one or more birefringent layers,wherein said birefringent layers comprise at least one of the following;an A-plate film, a C-plate film, or a biaxial film.
 28. The A-plateaccording to claim 27 wherein the optical axis of said A-plate issubstantially parallel to the sub-wavelength wires of said wire gridpolarization analyzer.
 29. The A-plate according to claim 27 wherein theoptical axis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization analyzer.
 30. Thesystem of claim 26 wherein said reflective liquid crystal device has avertically aligned construction.
 31. The apparatus of claim 26 whereinthe polarization axis orientation of said wire grid pre-polarizer isperpendicular to the polarization axis orientation of said wire gridpolarization analyzer.
 32. The apparatus of claim 26 wherein thepolarization axis orientation of said wire grid pre-polarizer isparallel to the polarization axis orientation of said wire gridpolarization analyzer.
 33. A display apparatus comprising: (a) a lightsource for forming a beam of light; (b) a wire grid pre-polarizer forpolarizing said beam of light to provide a polarized beam of light; (c)a wire grid polarizing beamsplitter for receiving said polarized beam oflight, for transmitting said polarized beam of light having a firstpolarization, and for reflecting said polarized beam of light having asecond polarization; (d) a reflective liquid crystal device forselectively modulating said polarized beam of light having a firstpolarization to encode image data thereon in order to form a modulatedbeam, and for reflecting said modulated beam back to said wire gridpolarizing beamsplitter, (e) a compensator, located between said wiregrid polarization beamsplitter and said reflective liquid crystaldevice, for conditioning oblique and skew rays of said modulated beam;(f) a wire grid polarization analyzer which removes residual unmodulatedfirst polarization light; and (g) image-forming optics for forming animage from said modulated beam.
 34. The system of claim 33 wherein saidcompensator comprises one or more birefringent layers, wherein saidbirefringent layers comprise at least one of the following; an A-platefilm, a C-plate film, or a biaxial film.
 35. The A-plate according toclaim 34 wherein the optical axis of said A-plate is substantiallyparallel to the sub-wavelength wires of said wire grid polarizationbeamsplitter.
 36. The A-plate according to claim 34 wherein the opticalaxis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization beam splitter. 37.The system of claim 33 wherein said reflective liquid crystal device hasa vertically aligned construction.
 38. The compensator of claim 33 whichmodifies polarization states of the oblique and skew rays relative tosaid wire grid polarization beamsplitter, or said reflective liquidcrystal device, or both.
 39. A display apparatus comprising: (a) a lightsource for forming a beam of light; (b) a wire grid pre-polarizer forpolarizing said beam of light to provide a polarized beam of light; (c)a wire grid polarizing beamsplitter for receiving said polarized beam oflight, for transmitting said polarized beam of light having a firstpolarization, and for reflecting said polarized beam of light having asecond polarization; (d) a reflective liquid crystal device forselectively modulating said polarized beam of light having a firstpolarization to encode image data thereon in order to form a modulatedbeam, and for reflecting said modulated beam back to said wire gridpolarizing beamsplitter; (e) a first compensator located between saidwire grid polarization beamsplitter and said reflective liquid crystaldevice, for conditioning oblique and skew rays of said modulated beam;(f) a wire grid polarization analyzer which removes residual unmodulatedfirst polarization light; (g) a second compensator for conditioningoblique and skew rays of said wire grid polarizing beamsplitter relativeto said wire grid polarization analyzer and said wire gridpre-polarizer; and (h) image-forming optics for forming an image fromsaid modulated beam.
 40. The system of claim 39 wherein said firstcompensator and said second compensator each comprise one or morebirefringent layers, wherein said birefringent layers comprise at leastone of the following; an A-plate film, a C-plate film, or a biaxialfilm.
 41. The system of claim 39 wherein said reflective liquid crystaldevice has a vertically aligned construction.
 42. The first compensatorof claim 39 which modifies polarization states of the oblique and skewrays relative to said wire grid polarization beamsplitter, or saidreflective liquid crystal device, or both.
 43. A method for projectingan image generated from image data, the method comprising: (a) providinga polarized light beam; (b) directing said polarized light beam to awire grid polarizing beamsplitter, said beamsplitter transmittingincident light having a first polarization as a transmitted beam, andreflecting incident light having a second polarization as a reflectedbeam; (c) modulating said transmitted beam from said wire gridpolarizing beamsplitter to encode image data at a reflective liquidcrystal device and to provide a modulated beam; (d) disposing acompensator in the path of said modulated beam to remove leakage lightfrom said modulated beam; and (e) projecting said modulated beam to formsaid image.
 44. A display apparatus comprising: (a) a light source forforming a beam of light; (b) a wire grid pre-polarizer for polarizingsaid beam of light to provide a polarized beam of light; (c) a wire gridpolarizing beamsplitter for receiving said polarized beam of light, fortransmitting said polarized beam of light having a first polarization,and for reflecting said polarized beam of light having a secondpolarization; (d) a reflective liquid crystal device for selectivelymodulating said polarized beam of light having a first polarization toencode image data thereon in order to form a modulated beam, and forreflecting said modulated beam back to said wire grid polarizingbeamsplitter; (e) a wire grid polarization analyzer which removesresidual unmodulated first polarization light; (f) a compensator forconditioning oblique and skew rays from said wire grid pre-polarizer andsaid wire grid polarization analyzer; and (g) image-forming optics forforming an image from said modulated beam.
 45. The apparatus of claim 44wherein said compensator comprises one or more birefringent layers,wherein said birefringent layers comprise at least one of the following;an A-plate film, a C-plate film, or a biaxial film.
 46. The A-plateaccording to claim 45 wherein the optical axis of said A-plate issubstantially parallel to the sub-wavelength wires of said wire gridpolarization beamsplitter.
 47. The A-plate according to claim 45 whereinthe optical axis of said A-plate is substantially perpendicular to thesub-wavelength wires of said wire grid polarization beamsplitter. 48.The apparatus of claim 44 wherein said reflective liquid crystal devicehas a vertically aligned construction.
 49. The apparatus of claim 44wherein said compensator is located between said wire grid pre-polarizerand said wire grid polarization beamsplitter.
 50. The apparatus of claim44 wherein said compensator is located between said wire gridpolarization beamsplitter and said wire grid polarization analyzer. 51.A display apparatus comprising: (a) a light source for forming a beam oflight; (b) a pre-polarizer for polarizing said beam of light to providea polarized beam of light; (c) a wire grid polarizing beamsplitter forreceiving said polarized beam of light, for transmitting said polarizedbeam of light having a first polarization, and for reflecting saidpolarized beam of light having a second polarization; (d) a reflectiveliquid crystal device for selectively modulating said polarized beam oflight having a first polarization to encode image data thereon in orderto form a modulated beam, and for reflecting said modulated beam back tosaid wire grid polarizing beamsplitter; (e) a wire grid polarizationanalyzer which removes residual unmodulated first polarization light;(f) image-forming optics for forming an image from said modulated beam;and (g) a compensator, located between said wire grid polarizingbeamsplitter and said reflective liquid crystal device for conditioningoblique and skew rays of said modulated beam.
 52. A display apparatuscomprising: (a) a light source for forming a beam of light; (b) apre-polarizer for polarizing said beam of light to provide a polarizedbeam of light; (c) a wire grid polarizing beamsplitter for receivingsaid polarized beam of light, for transmitting said polarized beam oflight having a first polarization, and for reflecting said polarizedbeam of light having a second polarization; (d) a reflective spatiallight modulator for selectively modulating said polarized beam of lighthaving a first polarization to encode image data thereon in order toform a modulated beam, and for reflecting said modulated beam back tosaid wire grid polarizing beamsplitter; (e) a compensator, locatedbetween said wire grid polarization beamsplitter and said reflectivespatial light modulator, for conditioning oblique and skew rays of saidmodulated beam; (f) wherein said wire grid polarizing beamsplitterreflects said compensated modulated beam; (g) a polarization analyzerwhich removes residual unmodulated first polarization light; and (h)image-forming optics for forming an image from said modulated beam.